1. Field of the Invention
The invention relates to a process for producing structures in the submicron range.
2. Description of Related Art
European Published Patent Application 0 388 484 discloses an aqueous-alkaline developable, highly resolving photoresist for producing structures in the submicron range. This resist consists of a developable base polymer and of a photoactive constituent (as well as possibly other customary additives), whereby the base polymer, as developable--and thus solutizing--groups, exhibits anhydride functions. The anhydride-containing base polymers, which are DUV-transparent and which are suited both for the single-layer and the two layer technique, preferably contain 10 to 55 mol % maleic anhydride and are obtained through co- or ter-polymerization with allyl trimethylsilane and styrol, as well as possible with another monomer, for example maleinimide.
When exposed to DUV, for example at a wavelength of 248 nm, photoresists with these types of base polymers, which as photoactive constituents contain diazo naphthoquinone derivatives or diazoketone derivatives, require a dose of about 70 to 80 mJ/cm.sup.2 for a zero bias exposure. This relatively high dose can be disadvantageous, because at a low throughput rate in production lines, for example, for highly integrated microelectronic components, the cost-intensive exposure units (steppers) have to be occupied for a long time; therefore, generally a sensitivity of &lt;20 mJ/cm.sup.2 is required for production. When silicon-containing resists of this type are applied using the two-layer technique, a problem also arises that after the structure transfer in the oxygen plasma, submicron structures are obtained in the bottom resist, which compared to the structures in the top resist have lost more than 10% of their line width. However, this cannot be tolerated in the production.
European Published Patent Application 0 395 917 discloses a photostructuring process, which allows submicron structures to be transferred photolithographically using the two-layer technique. In this process, the advantages of anhydride-containing base polymers or resists are utilized and, at the same time, a pattern transfer true to size is rendered possible. This means that the exact mask dimensions are reproduced in the bottom resist using the two-layer technique. This is accomplished by chemically expanding the anhydride-containing structures in the top resist to an extent that corresponds exactly to the loss in dimensions which occurs during the structure transfer in the oxygen plasma. In this process as well, a relatively high dose is needed to structure the top resist, namely about 90 mJ/cm.sup.2 (in the DUV range).
Furthermore, for the first time, the mentioned photostructuring process made it feasible to produce structure dimensions smaller than those stipulated by the physical resolution limit of the applied lithographic technique. This means that the width of the spaces in the photoresist structure is reduced in size to below the resolution limit. This is achieved by chemically expanding the anhydride-containing resist structures by a measure of a few nanometers up to a few micrometers, by which means the spaces are narrowed correspondingly. In this procedure, which is feasible both for the single-layer as well as for the two-layer technique, resists with diazoketone derivatives are applied as photoactive components. However, these resists do not satisfy production demands with respect to sensitivity.
U.S. Pat. No. 4,491,628 discloses a positive- or negative-working resist composition. Here, the problem of the inadequate sensitivity of conventional resists is approached with the concept of so-called chemical amplification. To this end, the resist composition contains a polymer with acid-labile residues and a photoinitiator, which releases an acid when irradiated. Base polymers are used which possess alkali-soluble groups, for example phenolic OH-groups (in a vinylphenol polymer), which are blocked by acid-cleavable groups, for example a tertiary butyl or a tertiary butoxycarbonyl group. Thus, the polymer is initially alkali-insoluble. As a photoinitiator (or photoacid), which forms a strong acid when irradiated with DUV light, or with electron or X-ray beams, so-called Crivello salts are used, that is onium salts, such as triphenylsulphonium-hexafluorophosphate.
The high sensitivity of resist compositions of the above mentioned type can be explained by the fact that during a temperature treatment following the exposure ("post exposure bake"), a single proton produced from the photoacid during exposure, and in fact by means of a photon, catalytically cleaves off multiple acid-cleavable groups, that is it releases a number of alkali-soluble groups. On the other hand, in the case of conventional resists containing diazo compounds (as a photoactive constituent), a maximum of only one single alkali-soluble acid grouping can be produced per photon. A sensitivity is specified for the resist compositions which ranges in the case of UV irradiation from 5 to 55 mJ/cm.sup.2 or, in the case of electron-beam irradiation, lies at 10 .mu.C/cm.sup.2. Moreover, they exhibit a high contrast. The disadvantage of these systems, however, is that a structure transfer true to size is not possible, and narrow spaces, that is spaces with a width smaller than the resolution limit, cannot be produced. In addition, either the DUV transparency or the etch resistance to substrate etching processes is relatively low.
From U.S. Pat. Nos. 4,837,124 and 4,912,018, a photoresist composition is known for DUV and excimer-laser lithography, which likewise works according to the chemical amplification concept. For that, the resist composition has 1 to 50% of a latent photoacid and 50 to 99% of a film-forming, imide-group containing polymer, dissolved together in a solvent. To make the polymer alkali-insoluble, an adequate quantity of the imide groups is blocked with acid-labile groups; such groups are oxycarbonyl groups, in particular the tertiary butoxycarbonyl group. The acid-labile groups are cleaved off in the described manner by means of the photoacid. As a result, the polymer--because of the developing imide groups--becomes alkali-soluble. The advantage of this resist composition is an improved resolution capacity, however, it has the same disadvantages as the resist composition according to U.S. Pat. No. 4,491,628. Moreover, it is disadvantageous that the introduction of the acid-labile groups by means of a polymer analogous reaction is not controllable and does not proceed with reproducibility and, consequently, also does not follow quantitatively.